The present invention relates to a damping force control type hydraulic shock absorber attached to a suspension system of a vehicle, for example, an automobile.
Hydraulic shock absorbers attached to suspension systems of automobiles or other vehicles include damping force control type hydraulic shock absorbers which are arranged such that the level of damping force can be properly controlled in accordance with the road surface conditions, vehicle running conditions, etc., with a view to improving the ride quality and the steering stability.
In general, this type of hydraulic shock absorber includes a cylinder having a hydraulic fluid sealed therein. A piston, which has a piston rod connected thereto to constitute a piston assembly, is slidably fitted in the cylinder to divide the inside of the cylinder into two chambers. The piston assembly is provided with a main hydraulic fluid passage and a bypass passage, which provide communication between the two chambers in the cylinder. The main hydraulic fluid passage is provided with a damping force generating mechanism including an orifice and a disk valve. The bypass passage is provided with a damping force control valve for controlling the flow path area of the bypass passage. It should be noted that a reservoir is connected, via a base value, to one of the chambers in the cylinder to compensate for a volumetric change in the cylinder due to extension and compression of the piston rod by the compression and expansion of a gas sealed in the reservoir.
With the above arrangement, when the bypass passage is opened by the damping force control valve, the flow resistance to the hydraulic fluid flowing between the two chambers in the cylinder is reduced, thereby reducing damping force. When the bypass passage is closed, the flow resistance between the two chambers is increased, thereby increasing damping force. Thus, damping force characteristics can be properly controlled by opening and closing the damping force control valve.
However, the above-described arrangement, in which damping force is controlled by changing the flow path area of the bypass passage, suffers from the problem that although the damping force characteristics can be changed to a considerable extent in the low piston speed region because damping force depends on the orifice area of the bypass passage, the damping force characteristics cannot greatly be changed in the intermediate and high piston speed regions because in these regions damping force depends on the damping force generating mechanism (disk valve, etc.; the constant level of the valve opening pressure) of the main hydraulic fluid passage.
To solve the above problem, there has heretofore been proposed a damping force control type hydraulic shock absorber in which a pressure chamber is formed at the back of a main valve serving as a damping force generating mechanism in a main hydraulic fluid passage which allows a hydraulic fluid to flow therethrough in response to the movement of a piston. The pressure chamber is communicated with a part of the main passage on the upstream side of the main valve through a fixed orifice and also communicated with a part of the main passage on the downstream side of the main valve through a variable orifice. This device is disclosed, for example, in Japanese Patent Application Unexamined Publication (KOKAI) No. 4-312227.
According to the above damping force control type hydraulic shock absorber, by opening and closing the variable orifice, the flow path area for hydraulic fluid is directly controlled, and the pressure in the pressure chamber is also changed. Thus, the valve opening initial pressure of the main valve can be changed. Accordingly, it is possible to simultaneously control orifice characteristics (in which damping force is approximately proportional to the square of the piston speed) and valve characteristics (in which damping force is approximately proportional to the piston speed), and hence possible to widen the control range for damping force characteristics.
In the above-described damping force control type hydraulic shock absorber, however, before the main valve opens, damping force due to orifice characteristics, which depends on the flow path area of the variable orifice, is generated. Therefore, if it is intended to ensure sufficient damping force in the low piston speed region, the damping force is likely to become excessively strong in the vicinity of the valve opening point of the main valve; if it is intended to obtain appropriate damping force in the vicinity of the main valve opening point, it is likely to become difficult to obtain sufficient damping force in the low piston speed region. Therefore, it is difficult to properly set damping force in the piston speed range from the low speed region to the vicinity of the main valve opening point.